This invention relates to a thermomechanical method and apparatus for measuring the concentration of a gas and in particular a reactive gas (i.e. gasses that may be reacted to produce by-products and which produce heat or remove heat from a system as a result of the reaction). Examples of such gasses include ozone, oxygen, nitrogen or oxides of nitrogen, and the like. The method and apparatus may function independent of the temperature of a gas mixture containing the reactive gas.
There are instruments available that measure ozone concentration in a gas mixture. One such instrument is disclosed in U.S. Pat. No. 5,167,927, Karlson. This Pat. discloses an apparatus that measures heat energy released when a reactive gas is catalytically converted to a different gas, for example using a catalyst to convert ozone to oxygen. In particular, Karlson discloses an apparatus that directs a stream of a gas mixture containing ozone against thermally conducting heat sinked plates on opposite sides of an axis of the stream with the plates extending upstream at an acute angle to the axis. One plate carries on its upstream facing a coating including a catalyst for ozone, while the other plate includes no catalyst on its upstream facing. A sensor is provided for measuring the temperatures of the respective plates. A separate chamber is provided having a similar arrangement of plates. The ozone concentration of the gas is electronically measured as the resistance difference of the two plates in each chamber.
One disadvantage of Karlson is that the flow of the gas mixture stream in Karlson is important to the instruments sensitivity, and is electronically controlled to be constant for each sample of gas mixture measured. A further disadvantage is that the instrument""s sensitivity and time constant is dependant on the velocity of the sample flow through the instrument, its electronic time constant and its thermal time constant.
There is also a need for an inexpensive, durable and easily calibrated apparatus for determining the concentration of reactive gasses such as ozone.
In accordance with the instant invention a thermochemical means is used to produce movement of a member which is drivingly connected to an indicator. In a preferred embodiment, the indicator is associated with an analog scale to thereby provide a readout of the concentration of a gas without the use of any electronic monitoring means. In this way a simple mechanical sensor for reactive gasses such as ozone and NOX can be produced.
In accordance with the instant invention, there is provided a method of measuring a change in heat produced during conversion of a reactive gas by a catalyst to produce a by-product, the method comprising the steps of:
(a) using a first temperature sensor which undergoes movement with changes in temperature to which it is exposed to produce. a first measurement representing the initial temperature of the gas mixture;
(b) exposing at least a portion of the gas mixture to the catalyst to produce a change in heat;
(c) using a second temperature sensor which undergoes movement with changes in temperature to which it is exposed to produce a second measurement representing the change in heat due to the portion of the gas stream being exposed to the catalyst; and,
(d) combining the first measurement and the second measurement to determine a differential measurement representing a measurement of the heat change during conversion of the reactive gas by the catalyst.
In one embodiment, the method further comprises the step of calibrating the first and second temperature sensors so that when there is no reactive gas within the gas mixture the first measurement and the second measurement produce a differential measurement which is a fixed measurement and is preferably zero.
In another embodiment, the differential measurement is calibrated to represent the concentration of the reactive gas within the gas mixture and the method further comprises the step of reading the concentration of the reactive gas from a display of the differential measurement.
In another embodiment, the first temperature sensor is positioned in a first chamber and the second temperature sensor is positioned in a second chamber and the method further comprises dividing the gas stream into two portions and introducing a portion into each chamber.
In another embodiment, the first temperature sensor is positioned in a first chamber and the second temperature sensor is positioned in a second chamber and the method further comprises sequentially passing the gas stream through the first chamber and then the second chamber.
In another embodiment, the method further comprises first converting. a specific gas in a gas stream to produce the reactive gas. Thus, the method may be used to measure the concentration of a non-reactive gas (eg. oxygen or nitrogen).
In another embodiment, the first measurement and the second measurement are signals produced by the temperature sensors and the method further comprises reading the first and second measurements prior to combining them to determine a differential measurement.
In another embodiment, the first measurement and the second measurement are opposed forces which are exerted on a member and the net movement of the member produces the differential measurement and the method further comprises reading the differential measurement.
In accordance with another aspect of the instant invention, there is also provided a method of measuring a change in heat produced during conversion of a reactive gas by a catalyst to a by-product, the method comprising the steps of:
(a) exposing the gas mixture to the catalyst to generate heat; and,
(b) using a temperature sensor which undergoes movement with changes in temperature to which it is exposed to produce a measurement representing the change in heat due to the gas stream being exposed to the catalyst.
In one embodiment, the method further comprises the step of calibrating the temperature sensor so that at ambient conditions when there is no reactive gas within the gas mixture the measurement is constant and preferably is zero. The measurement may be calibrated to represent the concentration of the reactive gas within the gas mixture and the method further comprises the step of reading the concentration of the reactive gas from a display of the measurement.
In another embodiment, the method further comprises first converting a specific gas in a gas stream to produce the reactive gas.
In another embodiment, the measurement is a force which is exerted on a member and the movement of the member produces a corresponding measurement and the method further comprises reading the corresponding measurement.
In accordance with another aspect of the instant invention, there is also provided an apparatus comprising:
(a) a catalyst positioned in an air flow path of a gas mixture containing a reactive gas, the reactive gas undergoing a reaction to produce a by-product and a change in heat upon exposure to the catalyst; and,
(b) a reacted gas temperature sensor which undergoes movement with changes in temperature to which it is exposed to produce a reacted gas measurement representing the change in heat produced by the reaction of the reactive gas.
In one embodiment, the measurement is calibrated to represent the concentration of the reactive gas within the gas mixture.
In another embodiment, the temperature sensor includes an indicator, the apparatus further comprises an analog display which is calibrated to represent the concentration of the reactive gas within the gas mixture, and the measurement comprises the movement of the indicator due to the change in temperature of the temperature sensor. Alternately, the measurement may be an electronic signal. However in this alternate embodiment, the sensing means still uses a thermomechanical member. The difference resides in the type of signal which is produced.
In another embodiment, the gas mixture comprises a specific gas and the apparatus further comprises a generator positioned upstream of the catalyst for converting at least a portion of the specific gas to the reactive gas.
In another embodiment, the apparatus further comprises:
(a) an ambient temperature chamber for receiving at least a portion of the gas mixture, the ambient temperature chamber having an ambient temperature sensor which undergoes movement with changes in temperature to which it is exposed to produce an ambient measurement representing the ambient temperature of the gas mixture;
(b) a second chamber in which the reacted gas temperature sensor is positioned; and,
(c) a differentiator to produce a differential measurement that represents a measurement of the heat generated during conversion of the reactive gas by the catalyst independent of the ambient temperature of the gas mixture containing the reactive gas.
The ambient and reacted gas temperature sensors may be calibrated so that when there is no reactive gas within the gas mixture the ambient measurement and the reacted gas measurement produce a differential measurement that is constant and, preferably is zero. Further, the differential measurement may be calibrated to represent the concentration of the reactive gas within the gas mixture. The apparatus may include a divider having two outlets, one of which is in air flow communication with the ambient chamber and the other of which is in air flow communication with the reacted gas chamber. Alternately, the apparatus may have an air inlet to the ambient chamber, a passageway connecting the chambers in air flow communication, and an air outlet from the reacted gas chamber, whereby the gas mixture passes sequentially through the ambient chamber and then the reacted gas chamber.
The measurements may be one or more electronic signals or they may be opposed forces which are exerted on the differentiator to produce a net movement of the differentiator which is the differential measurement. The apparatus may further comprise an analog display and an indicator associated with the analog display and drivienly connected to one of the temperature sensors or the differentiator.
In accordance with another aspect of the instant invention, there is also provided an apparatus comprising:
(a) a catalyst positioned in an air flow path of a gas mixture containing a reactive gas, the reactive gas undergoing a reaction to produce a by-product and a change in heat upon exposure to the catalyst;
(b) a reacted gas thermomechanical sensing means for producing a movement of at least a portion of the sensing means due to changes in temperature to which it is exposed; and,
(c) indicating means for providing a readout corresponding to the concentration of the reactive gas in the gas mixture.
In one embodiment, the gas mixture comprises a specific gas and the apparatus further comprises a means positioned upstream of the catalyst for converting at least a portion of the specific gas to the reactive gas.
In another embodiment, the apparatus further comprises an ambient thermomechanical sensing means for contacting at least a portion of the gas mixture which has not been exposed to the catalyst and producing movement of at least a portion of the sensing means due to changes in temperature to which it is exposed.
In another embodiment, the movement of each sensing means produces electronic signals which are combined by the indicating means to produce the readout.
In another embodiment, the movement of each sensing means produces opposed forces which cause the indicating means to move by a net amount to provide the readout which preferably provides a readout on an analog scale.
In another embodiment, the gas mixture comprises a specific gas and the apparatus further comprises a means positioned upstream of the catalyst for converting at least a portion of the specific gas to the reactive gas.